In addition to the other advise given: When better weather comes, and you have some time, equip yourself with various convenient abrasives (e.g., throw-away nail files, abrasive erasers, emory paper, etc) and some electronic contact cleaner, and start going through the car cleaning every contact and connector you can reach. Sometimes, just pulling the connector apart and pushing it back together a couple of times will give you a clean contact point. Also have on hand some good quality electrical tape and a paint-on or spray-on insulation material. You'll never know how many midnight breakdowns and frustrating malfunctions this will prevent, because the ones due to poor contact will not happen. Also a good learning tool about where things are and how they relate, if you have the wiring diagram along and take a few extra minutes to check it from time to time.
--
We have met the enemy and they is us. [Pogo] '99 S70 cop car : Rough Rider tires& suspension, Walmart fog lights, speakers by ebay, ambiance by Pall Mall, trim by Le Duc d'Tape

The very first ground I would check is the battery. I have seen battery cable grounds corruded inside the sheathing where you never see it. Gives all kinds of electrical problems. I would replace the cable. They're cheap.

See the 700/900 FAQ under "electrical circuits" for full information.